A polyurethane prepolymer having terminal isocyanate groups is used for a one-pack type composition curable by reaction with moisture in the air and for a two-pack type composition curable by being mixed with an active hydrogen compound. Such a one-pack type or two-pack curable composition is widely used in the field of building materials or construction materials, such as sealing materials, waterproof materials or floor materials. Reflecting diversification of building and construction fashions in recent years, sealing materials and waterproof materials having higher flexibility and elongation are desired.
In order to obtain a cured product having flexibility and high elongation by a curable polyurethane composition, it is common to employ a method of using a polyoxyalkylenepolyol having a high molecular weight, or a method wherein the content of a bifunctional polyoxyalkylenepolyol is increased so that the average number of functional groups of the polyoxyalkylenepolyol component is decreased. However, such methods have drawbacks that the curing speed tends to be slow, and the tackiness of the cured products tends to increase, and they failed to provide satisfactory properties.